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What is West Nile virus?
West Nile virus is one of a group of viruses called arboviruses that are spread by mosquitoes and may cause illness in birds, animals, and humans. West Nile virus was not known to be present in the United States until the summer of 1999. Previously, West Nile virus was only found in Africa, western Asia, the Middle East, and Eastern Europe.
Where is West Nile virus in the United States?
West Nile virus was first identified as a disease threat in the United States during the summer of 1999 and was limited to the northeastern states through 2000. However, the virus rapidly expanded its geographic range. By the end of 2004, West Nile virus had spread from the Atlantic to the Pacific coast with viral activity confirmed in all 48 contiguous states.
How is it spread?
West Nile virus is primarily spread through the bite of an infected mosquito. Mosquitoes pick up the virus when they feed on infected birds. The virus must then circulate in the mosquito for a few days before they are capable of passing the infection to animals or humans while biting. The virus is found in the salivary gland of the mosquito. During feeding, the virus may be injected into a human or animal where it may multiply and possibly cause disease.
How long does it take to get sick after a bite from an infected mosquito?
It takes about three to 15 days for both human and equine (horse, mule, or donkey) illness to occur.
What are the symptoms in humans?
Most persons who are infected with West Nile virus will have no noticeable symptoms, or have an illness syndrome called “West Nile Fever” lasting two to ten days. Common symptoms of West Nile Fever include headache, fever, and extreme muscle weakness, occasionally accompanied by vomiting or skin rashes. In some cases, West Nile virus infection will cause severe neurologic disease such as meningitis (swollen membranes in brain and spinal cord), paralysis (loss of muscle function), or encephalitis (swelling and inflammation of the brain). Symptoms of West Nile meningitis or encephalitis may be intense headache, dizziness, stiff neck, marked weakness, muscle tremors, dis-orientation, mental confusion, or convulsions.
Who is at greatest risk?
Individuals are at risk of acquiring West Nile virus infection if they travel to or live in an area where virus transmission is known to occur. People whose work and hobbies take them outdoors for extended periods of time are at increased risk of being infected due to the increased exposure to mosquitoes. Persons over the age of 50 years appear to be at greatest risk of developing severe disease of the nervous system.
How is West Nile disease treated?
There is no specific treatment. Severe cases often require hospitalization and intensive supportive care.
Do all birds infected with West Nile virus die?
No. The severity of the disease in birds appears to vary with the species of birds infected. Birds belonging to the Corvidae family (crows, jays, magpies, and ravens) appear to be very sensitive to West Nile viral infection and suffer a high death rate. Although mortality in a wide variety of birds has been a hallmark of West Nile virus in the U.S., many species of birds tend to survive West Nile virus infection.
What animals besides birds and horses may develop illness from West Nile virus infection?
Birds and equine animals (horses, donkeys, mules, zebras) are most susceptible to disease caused by West Nile virus. Although numerous animals are bitten by infected mosquitoes, illness caused by West Nile virus is uncommon in domestic pets or other wildlife. West Nile virus infection coupled with disease has been rarely diagnosed in a variety of animals, including squirrels, bats, dogs, cats, goats, skunks, domestic rabbits, and even alligators. However, West Nile virus is NOT considered a significant health threat for dogs and cats.
Continued on other side
West Nile Virus
Public Health
Fact Sheet
Public Health
Fact Sheet

What is West Nile virus?
West Nile virus is one of a group of viruses called arboviruses that are spread by mosquitoes and may cause illness in birds, animals, and humans. West Nile virus was not known to be present in the United States until the summer of 1999. Previously, West Nile virus was only found in Africa, western Asia, the Middle East, and Eastern Europe.
Where is West Nile virus in the United States?
West Nile virus was first identified as a disease threat in the United States during the summer of 1999 and was limited to the northeastern states through 2000. However, the virus rapidly expanded its geographic range. By the end of 2004, West Nile virus had spread from the Atlantic to the Pacific coast with viral activity confirmed in all 48 contiguous states.
How is it spread?
West Nile virus is primarily spread through the bite of an infected mosquito. Mosquitoes pick up the virus when they feed on infected birds. The virus must then circulate in the mosquito for a few days before they are capable of passing the infection to animals or humans while biting. The virus is found in the salivary gland of the mosquito. During feeding, the virus may be injected into a human or animal where it may multiply and possibly cause disease.
How long does it take to get sick after a bite from an infected mosquito?
It takes about three to 15 days for both human and equine (horse, mule, or donkey) illness to occur.
What are the symptoms in humans?
Most persons who are infected with West Nile virus will have no noticeable symptoms, or have an illness syndrome called “West Nile Fever” lasting two to ten days. Common symptoms of West Nile Fever include headache, fever, and extreme muscle weakness, occasionally accompanied by vomiting or skin rashes. In some cases, West Nile virus infection will cause severe neurologic disease such as meningitis (swollen membranes in brain and spinal cord), paralysis (loss of muscle function), or encephalitis (swelling and inflammation of the brain). Symptoms of West Nile meningitis or encephalitis may be intense headache, dizziness, stiff neck, marked weakness, muscle tremors, dis-orientation, mental confusion, or convulsions.
Who is at greatest risk?
Individuals are at risk of acquiring West Nile virus infection if they travel to or live in an area where virus transmission is known to occur. People whose work and hobbies take them outdoors for extended periods of time are at increased risk of being infected due to the increased exposure to mosquitoes. Persons over the age of 50 years appear to be at greatest risk of developing severe disease of the nervous system.
How is West Nile disease treated?
There is no specific treatment. Severe cases often require hospitalization and intensive supportive care.
Do all birds infected with West Nile virus die?
No. The severity of the disease in birds appears to vary with the species of birds infected. Birds belonging to the Corvidae family (crows, jays, magpies, and ravens) appear to be very sensitive to West Nile viral infection and suffer a high death rate. Although mortality in a wide variety of birds has been a hallmark of West Nile virus in the U.S., many species of birds tend to survive West Nile virus infection.
What animals besides birds and horses may develop illness from West Nile virus infection?
Birds and equine animals (horses, donkeys, mules, zebras) are most susceptible to disease caused by West Nile virus. Although numerous animals are bitten by infected mosquitoes, illness caused by West Nile virus is uncommon in domestic pets or other wildlife. West Nile virus infection coupled with disease has been rarely diagnosed in a variety of animals, including squirrels, bats, dogs, cats, goats, skunks, domestic rabbits, and even alligators. However, West Nile virus is NOT considered a significant health threat for dogs and cats.
Continued on other side
West Nile Virus
Public Health
Fact Sheet
Public Health
Fact Sheet